The present invention relates to improvements in electro-optic modulators. In particular, the present invention relates to methods and apparatus for reducing bias point sensitivity to ambient temperature and applied RF in an electro-optic modulator.
Electro-optic modulators are typically biased with a DC voltage to set the quiescent phase difference between the two optical paths and to establish the operating point on the intensity-voltage curve about which modulation is induced. The bias point of electro-optic modulators is a function of the ambient temperature and the applied RF. As the ambient temperature and the applied RF changes, the desired bias point changes. The sensitivity of the bias point to ambient temperature and to applied RF can cause an increase in the bit error rate in digital communication systems.
FIG. 1 illustrates a schematic diagram of a prior art asymmetric co-planar waveguide (ACPW) Mach-Zehnder Interferometric (MZI) modulator indicating the field lines and thermal stress from the comers of the electrodes. Asymmetric RF electrodes are used to produce chirped optical signals. The modulator 100 includes an electro-optic substrate 102 with two waveguides 104, 104xe2x80x2 diffused in the substrate 102. Crystal axes for x-cut lithium niobate are shown. A buffer layer 106 is formed on top of the substrate 102 and the two waveguides 104, 104xe2x80x2. An asymmetric co-planar waveguide (ACPW) electrode structure 108 is formed on top of the buffer layer 106. The electrode structure 108 includes a ground electrode 110 and a RF electrode 112.
Electric field lines 114 are illustrated for the electrode structure 108. The path of the electric field lines to waveguide 104 is significantly longer than the path of the electric field lines to waveguide 104xe2x80x2. Therefore, the modulation experienced by waveguide 104 is significantly weaker than the modulation experienced by waveguide 104xe2x80x2. The imbalance in modulation generates chirp, which can be desirable for some communication systems.
Asymmetric co-planar waveguide modulators have particularly strong bias point sensitivity to temperature. The bias point sensitivity results from a mismatch in thermal-expansion coefficients between the metal forming the electrodes, which is typically gold, and the electro-optic substrate, which is typically lithium niobate. The mismatch results in thermal stress 116 in the substrate 102 that is localized near the bottom comers of the electrodes as illustrated in FIG. 1. This xe2x80x9cthermal stressxe2x80x9d is a mechanical stress that is a function of temperature. The thermal stress 116 generates a piezoelectric voltage that is experienced by waveguide 104xe2x80x2.
The relatively wide ground electrode 110 causes significantly more thermal stress than the RF electrode 112 because it has a larger amount of strain accumulated across the width of the electrode and, therefore, generates a higher piezoelectric voltage compared with the RF electrode 112. The difference in the piezoelectric voltages experienced by waveguides results in a significant phase change that shifts the bias point of the modulator 100 as ambient temperature is increased.
Asymmetric co-planar waveguide modulators also have bias point sensitivity to the applied RF because of the xe2x80x9cskin-effect.xe2x80x9d The RF electrode 112 is significantly smaller in cross section than the ground electrode 110, and therefore introduces more RF attenuation than the ground electrode 110. The lost RF energy is dissipated as heat, which causes a rise in temperature in the waveguides. Since the ground electrode 110 is a more effective heat sink than the RF electrode 112, a temperature differential may be created between the waveguides 104, 104xe2x80x2. The temperature differential shifts the bias point because the waveguides 104, 104xe2x80x2 experience different magnitudes of thermal stress and because the optical refractive index of the substrate 102 changes as a function of temperature.
Some prior art electro-optic modulator designs use electrode structures that reduce bias point sensitivity to the applied RF signal. For example, xe2x80x9cLiNbO3 Mach-Zehnder Modulators with Fixed Negative Chirp,xe2x80x9d IEEE Photonics Technology Letters, Vol. 8, October 1996, pp. 1319-1321, describes various designs for x-cut lithium niobate chirped-modulator that reduce bias point sensitivity to the applied RF signal. Two of these prior art designs are illustrated below in FIG. 2 and FIG. 3.
FIG. 2 illustrates a schematic diagram of a prior art three electrode co-planar-waveguide Mach-Zehnder Interferometric modulator 130 having asymmetric gaps 132 that introduce chirp, yet reduce bias sensitivity to applied RF. The modulator 130 includes an electro-optic substrate 102 with two waveguides 104, 104xe2x80x2 diffused in the substrate 102. Crystal axes for x-cut lithium niobate are shown. A buffer layer 106 is formed on top of the substrate 102 and the two waveguides 104, 104xe2x80x2. Two ground electrodes 110 are formed on top of the buffer layer 106. A RF electrode 112 is formed on top of the buffer layer and it is asymmetrically positioned between the two ground electrodes 110.
FIG. 3 illustrates a schematic diagram of a prior art three electrode co-planar-waveguide Mach-Zehnder Interferometric modulator 140 having asymmetric waveguide locations that introduce chirp, yet reduce bias sensitivity to applied RF. The modulator 140 includes an electro-optic substrate 102 with two waveguides 104, 104xe2x80x2 diffused in the substrate 102. Crystal axes for x-cut lithium niobate are shown. A buffer layer 106 is formed on top of the substrate 102 and the two waveguides 104, 104xe2x80x2. Two ground electrodes 110 are formed on top of the buffer layer 106 so that they are asymmetrically positioned relative to the waveguides 104, 104xe2x80x2. A RF electrode 112 is formed on top of the buffer layer and it is symmetrically positioned between the two ground electrodes 110.
Although some prior art electrode structures for electro-optic modulator reduce the bias point sensitivity to applied RF, they do not reduce the bias point sensitivity to ambient temperature. This is because they do not relieve or compensate for stresses caused by thermal expansion resulting from changes in the ambient temperature.
The present invention relates to electro-optic modulators with reduced bias point sensitivity to ambient temperature and to applied RF. The modulators may be chirped or zero-chirp modulators. The invention is particularly useful for electro-optic modulators that have asymmetric co-planar waveguide electrode structures, which have relatively strong bias point sensitivity to ambient temperature and applied RF.
An electro-optic modulator of the present invention reduces the bias point sensitivity to ambient temperature and to applied RF by reducing the net phase shift caused by changes in the ambient temperature and by the applied RF field. Specifically, in one embodiment, an electro-optic modulator according to the present invention reduces the net phase shift by reducing the piezoelectric voltage experienced by one of the waveguides relative to the other waveguide. In another embodiment, an electro-optic modulator according to the present invention reduces the net phase shift by substantially matching the thermal stresses experienced by the waveguides and thus by causing the piezoelectric voltage experienced by one waveguide to be similar the piezoelectric voltage experienced by the other waveguide.
A discovery of the present invention is that bias point sensitivity in electro-optic modulators to both ambient temperature and to applied RF can be reduced by positioning the waveguides relative to the electrodes so that the thermal expansion proximate to one waveguide is similar to the thermal expansion proximate to the other waveguide. In one embodiment, an electro-optic modulator of the present invention reduces the net phase shift by positioning one waveguide proximate to the edge of the ground electrode.
Another discovery of the present invention is that net phase shift in an electro-optic modulator can be reduced by using an electrode structure that reduces the thermal stress. In one embodiment, an electro-optic modulator of the present invention includes a ground electrode that comprises thermal stress-relieving slots that reduce the generated piezoelectric voltage. In another embodiment, an electro-optic modulator of the present invention includes a ground electrode that has a relatively narrow-width.
Another discovery of the present invention is that net phase shift in an electro-optic modulator can be reduced by using a thermal stress compensation structure. In one embodiment, an electro-optic modulator of the present invention includes an in-line bias electrode that has a thermal sensitivity, which is opposite to the thermal sensitivity of the RF electrodes. In another embodiment, an electro-optic modulator of the present invention includes an asymmetric bias electrode with one wide ground electrode to provide a temperature sensitivity that is opposite in sign to that of the RF electrode. In another embodiment, an electro-optic modulator of the present invention includes a dielectric material that substantially matches thermal stress experienced by the two waveguides.
Accordingly, the present invention features a co-planar waveguide interferometric electro-optic modulator that may be a chirped modulator. The modulator includes a first and second waveguide that are formed in an electro-optic substrate. A RF electrode is positioned on the electro-optic substrate between the first and the second waveguide. In one embodiment, the RF electrode is asymmetrically positioned between the first and the second waveguide. A ground electrode is positioned proximate to the second waveguide. In one embodiment, the ground electrode includes at least one slot that reduces strain accumulated across the width of the ground electrode.
A width of the ground electrode relative to a width of the RF electrode is dimensioned to reduce a net phase shift of the modulator as a function of ambient temperature and, therefore, reduces the bias point sensitivity of the modulator to ambient temperature. In one embodiment, the width of the ground electrode is substantially less than ten times the width of the RF electrode.
In one embodiment, the modulator includes a dielectric material that is positioned proximate to the first waveguide and that causes a thermal stress that reduces the bias point sensitivity of the modulator to ambient temperature. In another embodiment, the modulator includes an electrode that is positioned proximate to the first waveguide that creates a thermal stress in the first waveguide that substantially matches an electrode-induced thermal stress in the second waveguide.
The present invention features another co-planar waveguide interferometric electro-optic modulator that includes a guard ground electrode that sinks heat from the RF electrode. The modulator may be a chirped modulator. The modulator includes a first and second waveguide that are formed in an electro-optic substrate. A RF electrode is positioned on the electro-optic substrate between the first and the second waveguide. In one embodiment, the RF electrode is asymmetrically positioned between the first and the second waveguide. A ground electrode is positioned proximate to the second waveguide.
A guard ground electrode is positioned proximate to the first waveguide. The guard ground electrode sinks heat that is generated by the RF electrode and, therefore, reduces the bias point sensitivity of the modulator to applied RF. In one embodiment, at least one of the ground electrode or the guard ground electrode includes at least one slot that reduces strain accumulated across the width of the ground electrode. In one embodiment, the modulator includes a dielectric material that is positioned proximate to the first waveguide and that causes a thermal stress that reduces the bias point sensitivity of the modulator to ambient temperature.
The present invention features another co-planar waveguide interferometric electro-optic modulator that includes a guard ground electrode that sink heat from the RF electrode and that also balances the thermal stress in the two waveguides. The modulator may be a chirped modulator. The modulator includes a first and second waveguide that are formed in an electro-optic substrate. A RF electrode is positioned on the electro-optic substrate between the first and the second waveguide. In one embodiment, the RF electrode is asymmetrically positioned between the first and the second waveguide. A ground electrode is positioned proximate to the second waveguide. In one embodiment, the ground electrode includes at least one slot that reduces strain accumulated across a width of the electrode.
A guard ground electrode is positioned proximate to the first waveguide. The guard ground electrode sinks heat that is generated by the RF electrode and, therefore, reduces the bias point sensitivity of the modulator to applied RF. The guard ground also causes the first waveguide to experience a thermal stress similar to the thermal stress experienced by the second waveguide and, therefore, reduces the bias point sensitivity of the modulator to ambient temperature. In one embodiment, at least one of the ground electrode or the guard ground electrode includes at least one slot that reduces strain accumulated across the width of the ground electrode.
In one embodiment, the guard ground electrode is positioned to cause the first waveguide to experience a thermal stress that substantially matches a thermal stress experienced by the second waveguide. In another embodiment, the guard ground electrode is dimensioned to cause the first waveguide to experience a thermal stress that substantially matches a thermal stress experienced by the second waveguide.
The present invention features another co-planar waveguide interferometric electro-optic modulator that includes a dielectric material to balance thermal stress in the two waveguides. The modulator may be a chirped modulator. The modulator includes a first and second waveguide that are formed in an electro-optic substrate. A RF electrode is positioned on the electro-optic substrate between the first and the second waveguide. In one embodiment, the RF electrode is asymmetrically positioned between the first and the second waveguide. A ground electrode is positioned proximate to the second waveguide.
A dielectric material is positioned proximate to the first waveguide. The dielectric material causes the first waveguide to experience a thermal stress that is similar to the thermal stress experienced by the second waveguide and, therefore, reduces the bias point sensitivity of the modulator to ambient temperature. In one embodiment, the dielectric material is positioned to cause the first waveguide to experience a thermal stress that substantially matches a thermal stress experienced by the second waveguide. In another embodiment, the dielectric material is dimensioned to cause the first waveguide to experience a thermal stress that substantially matches a thermal stress experienced by the second waveguide.
The present invention features an asymmetric co-planar waveguide Mach Zehnder interferometric electro-optic modulator that may be a chirped modulator. The modulator includes a first and second waveguide that are formed in a lithium niobate substrate. AN RF electrode is asymmetrically positioned on the lithium niobate substrate between the first and the second waveguide. A ground electrode is positioned proximate to the second waveguide. In one embodiment, the ground electrode has a width that is less than ten times a width of the RF electrode.
A guard ground electrode is positioned proximate to the first waveguide. The guard ground electrode sinks heat that is generated by the RF electrode and also causes the first waveguide to experience a thermal stress similar to the thermal stress experienced by the second waveguide. The modulator, therefore, reduces bias point sensitivity of the modulator to both ambient temperature and to applied RF.
The present invention also features a co-planar waveguide interferometric electro-optic modulator that includes a bias electrode modulator section. The bias electrode modulator section includes a first and second waveguide that are formed in an electro-optic substrate. A bias electrode is positioned on the electro-optic substrate between the first and the second waveguide. A first and second ground electrode are positioned on the electro-optic substrate proximate to the first and second waveguide, respectively. In one embodiment, at least one of the bias electrode, first electrode, and second electrode are positioned directly on the substrate. The bias electrode modulator section has a thermal sensitivity that is opposite to a thermal sensitivity of an RF electrode of the electro-optic modulator and, therefore, reduces the bias point sensitivity of the electro-optic modulator to ambient temperature.